Process for the production of pure indium



Aug. 14, 1962 w. MORAWIETZ PROCESS FOR THE PRODUCTION OF PURE INDIUMFiled June 26, 1961 INVENTOR.

' Wilhelm Morawiezz nited States atent 3,049,478 PROCESS FOR THEPRODUCTION OF PURE INDIUM Wilhelm Morawietz, Duisburg, Germany, assignort Duisburger Kupt'erhiitte, Duisburg, Germany, a German corporationFiled June 26, 1961, Ser. No. 119,434 Claims priority, applicationGermany July 12, 1960 22 Claims. (Cl. 204-64) The present inventionrel-ates to a process for the continuous electrolytic production ofmetallic indium of the highest purity from indium salt solutions, andmore particularly to such a process wherein a reflux is employed toenhance the purity of yield of indium produced by the overall process.

Present-day requirements demand metallic indium of the highest purity.Highest purity indium is especially desired for the production ofsemiconductors. As is known, indium is capable of being electrolyticallydeposited from its aqueous salt solutions at the cathode of anelectrolysis cell. Such electrolytic deposition, as a rule, is coupledwith a purifying effect with respect to the cathodic indium, theimpurities remaining in the electrolyte. This purifying effect isfurther aided by the fact that impure indium can be freed from a part ofits impurities by subjecting the impure metal to anodic dissolution inan electrolytic cell, whereby the impurities either remain on the anodeor pass into the electrolyte, such that upon reversing the polarity ofthe system subsequent cathodic deposition of indium may take placewithout contamination of the impurities present on the anode orremaining in the electrolyte.

It has been recognized that the effect of the purifying process ofindium by means of re-electrolysis can be improved by carrying out thepurifying process in several successive stages, whereby the cathodicindium obtained in a prior stage is subjected to anodic dissolution inthe next stage. As a consequence thereof, an electrolytic step-by-stepimprovement in the purity of indium is achieved.

One of the major difliculties which occurs in the electrolyticdeposition of indium from aqueous solutions where comparatively greaterthicknesses of sediment are to be deposited, usually on solidelectrodes, is that the indium is deposited at the cathode unevenly. Theindium deposit builds up in the form of so-called dendrites, such thatthe cathode metal extends toward the anode over comparatively shortperiods of electrolytic operation. Consequently, a short circuit betweenthe opposing anode and cathode is soon caused and the electrolysis isinterrupted.

The accompanying diificulties which occur on account of the dendriticdeposition of indium, at the cathode, can be overcome in theintermediate stages of a multi-stage electrolysis by not depositing theindium in its pure form in the first and in all the subsequent stagessave the last and by first placing mercury at the cathode as an alloyingelement. The reason for employing mercury at the cathode is due to thefact that indium is soluble in mercury to a high degree such that theindium can be later dissolved out or separated anodically andselectively from the indium amalgam, obtained, cathodical-ly, in view ofthe great difference in the normal potential between indium and mercurybeing subjected to electrolytic conditions.

While the use of mercury may avoid the formation of dendrites in all butthe last stage of electrolysis, the formation of dendrites at thecathode of the final deposition stage, however, and the difiicultiescaused thereby remain quite undirninished since mercury is, of course,not used in the final stage. Attempts have been made to overcome suchdifiiculties by choosing a suitable electrolyte'in the final stage. Theavoidance of these difliculties, nevertheless, cannot be. achievedsuccessfully where his sought to deposit comparatively larger amounts ofpure'indium in the final stage.

One of the usual disadvantages attending the conventional processes forthe electrolytic production of purest indium is the fact that theimpurities, which are primarily introduced into the "electrolysis withthe impure indium, become enriched in the electrolyte, even in theelectrode amalgam of the intermediate stage, and hence increasinglycontaminate the final product during the course of operation. As aresult, after a more or less short period of time, electrolysis has tobe terminated and the electrolyte in the final stage replaced.Therefore, indium salt or indium metal of the highest purity must beused for the production of the electrolyte in the finalstage in orderthat contamination from this source will not aggravate the conditioncaused by the enrichment of impurities from the impure indium startingmaterial used. Understandably, because of the foregoing disadvantages,purest indium has only been produced by conventional techniques in smalllaboratory amounts with poor yields regarding the amount of crude metalintroduced as starting material in the electrolysis.

In a special arrangement for the electrolytic purification of metalswith the aid of intermediate amalgam stages, in order to produceprimarily pure indium, static amalgam electrodes have been used whichserve the func tion of a cathode in the first stage and simultaneouslythe function of an anode in the next stage in succession. The indiummust pass from the cathode surface of one stage to the anode surface ofthe next stage by diifusion through the amalgam. Disadvantageously, inthe course of this diffusion, undesirable polarization phenomena occurwhich lessen the purifying effect of the electrolysis, unless thearrangement is operated with only a very low current density andcorrespondingly low rate of yield of the metal to be obtained.

In another arrangement, also using amalgam as an intermediate stage, theindium is deposited at a fixed cathode in the final stage.

None of the foregoing conventional processes have actually solved theproblem of purification of the electrolyte in the final stage and of theavoidance of ,formation of dendrites at the cathode in such a way thatthe continuous production of purest indium material can be carried outeffectively and in large yields.

It is an object of the present invention to overcome the foregoingdisadvantages and to provide a process for the continuous electrolyticproduction of metallic indium of the highest purity from indium saltsolutions in amounts significantly higher than the laboratory amountsheretofore obtained, and more economically than was heretofore the case.

Other and further objects of the invention will become apparent from astudy of the within specification and accompanying drawing, in which theFIGURE represents .a schematic view of an electrolytic cell arrangement,including a series of successively connected cells for carrying out theelectrolytic purification of indium metal against a reflux ofelectrolyte and mercury,

It has been found in accordance with the present invention that aprocess for the continuous electrolytic production of metallic indium ofthe highest purity from indium salt solutions may be providedeconomically and with comparatively high yields of pure indium metal.The process comprises electrolyzing an impure indium salt solution in afirst-stage electrolysis using mercury as cathode whereby to form afirst amalgam of mercury and electrolytically separated metallic indium,passing said amalgam to an intermediate stage electrolysis for 3electrolysis therein, using an indium salt solution as electrolyte, saidamalgam as anode and additional mercury as cathode, whereby to dissolveelectrolytically said metallic indium from said amalgam into saidelectrolyte and in turn electrolytically deposit said indium in saidadditional mercury as cathode to form further amalgam, passingelectrolyte from said intermediate stage electrolysis to saidfirst-stage electrolysis for combining with said impure salt solutionfor further electrolysis therein, passing mercury remaining uponelectrolytic removal of metallic indium from said first amalgam as anodefrom said intermediate stage to said first stage for use therein ascathode, passing said further amalgam to a last-stage electrolysis usingsaid further amalgam as anode, molten indium salt as electrolyte andmolten pure indium metal as cathode for dissolving electrolytically saidmetallic indium from said further amalgam into said molten indium saltand in turn electrolytically depositing said indium in said moltenindium metal as cathode, and thereafter recovering pure indium metalfrom said last stage.

Preferably, the electrolyte in the first and intermediate stages isindium halide solution and that in the last stage is molten water-freeindium halide. The indium halide may be indium chloride, indium bromide,indium iodide, or indium fluoride. Advantageously, the last stageelectrolyte is maintained at a temperature above its own fusion pointand above the fusion point of indium. The last stage electrolyte maycomprise a low-melting mixture of indium halides including one or moreof indium chloride, indium bromide, indium iodide, and indium fluoride.The last-stage electrolyte may additionally include an alkali halide, analkali earth halide, or a mixture of one or more of these halides.

The electrolyte is preferably agitated in the intermediate stage inorder to more effectively transfer indium passing from the anode amalgamto the cathode mercury for electrolytically effecting the flow throughthe succession of stages. In the same way, the amalgam is pumped fromone stage to the other in a lively manner for increasing the rate ofexchange.

The electrolyte of the first stage may be produced in accordance withone feature of the invention by anodic dissolution of impure indiummetal. In accordance with a further feature of the invention,electrolyte is added to the intermediate stage in the form of aqueoushydrohalic acid, such as hydrochloric acid, hydrobromic acid, hydroiodicacid, and hydrofluoric acid. Such acid preferably has a concentration of3-8 normal. In this connection, mercury may be added to the intermediatestage substantially free from indium for passage against theelectrolytic flow of indium being purified.

In the case of the electrolyte in the last stage, the same may theproduced, if desired, by the action of a halogen gas or a hydrogenhalide gas on the further amalgam passed to the last stage. In this way,the indium halide salt solution is formed in molten water-freecondition. The particular gas used may be rarefied by an inert gas orhydrogen, if desired.

In accordance with a particular feature of the invention, thefirst-stage electrolysis includes at least two individual electrolysiscells connected in series, the electrolyte and amalgam being passedthrough the series of cells in this stage in opposite directions. Inthis manner, a more effective utilization of the mercury forcathodically removing indium from the electrolyte may be carried out.

In accordance with a preferred embodiment of the invention, therefore, aprocess is provided for the continuous electrolytic production ofmetallic indium of highest purity from indium salt solutions, whichcomprises electrolyzing an impure indium salt solution being passed to afirst-stage electrolysis using mercury as cathode whereby to form afirst amalgam of mercury and electrolytically separated metallic indium,passing said amalgam to an intermediate stage electrolysis forelectrolysis therein using an indium salt solution as electrolyte, saidfirst amalgam as anode and additional mercury as cathode whereby todissolve electrolytically said metallic indium from said first amalgaminto said electrolyte and in turn electrolytically deposit at least aportion of said indium in said additional mercury as cathode formingfurther amalgam, passing a portion of said indium salt solution enrichedby a portion of said metallic indium dissolved electrolytically thereinfrom said intermediate stage electrolysis back to said first stageelectrolysis for combining with said impure indium salt solution forfurther electrolysis therein, passing mercury remaining uponelectrolytic removal in said intermediate stage electrolysis of metallicindium from said first amalgam as anode from said intermediate stageback to said first stage for use therein as cathode, passing saidfurther amalgam to a laststage electrolysis using said further amalgamas anode, molten indium salt as electrolyte, and pure molten indiummetal as cathode for dissolving electrolytically said metallic indiumfrom said further amalgam into said molten indium salt and in turnelectrolytically depositing said indium in said molten indium metal ascathode, recovering a portion of pure indium metal corresponding to thatproduced in said electrolysis from said last stage, recovering a portionof electrolyte from said first stage, adding fresh electrolyte to saidintermediate stage to replace that passed to said first stage andcorresponding to that recovered from said first stage, recovering aportion of mercury being passed from said intermediate stage to saidfirst stage, and adding fresh mercury to said intermediate stage,corresponding to that recovered from said first stage.

Accordingly, the present invention provides for the removal ofimpurities from an impure indium starting material by mechanicallydirecting a flux or flow of electrolyte and electrode mercury againstthe conveyance of indium to the cathode of the final deposition stage bythe electrolytic passage of indium successively through the variousstages. The flux or flow carries back the impurities along with aportion of the indium being subjected to electrolytic action andeventually removes these impurities from the process. The dissolutionand deposition steps occurring at the mercury and amalgam electrodes areeffected with small phase passage potentials and, hence, with a highselectivity for the Nernst potentials of the metals present in theelectrode material as well as in the electrolyte.

It will be appreciated that where only one stage of electrolysis isemployed, it is necessary to remove from the process a considerableminimum amount of indium in reflux in order to attain desirably acertain degree of purity of the metal finally recovered. Where severalstages of electrolysis are used in succession, on the other hand, sothat in each stage the indium is anodically dissolved and cathodicallydeposited again, such that the metal cathodically obtained is used forthe anodic dissolution in the next stage, the amount of indium whichmust be recirculated by means of the reflux flow and finally removedfrom the electrolysis may be reduced yet the same purifying effectachieved if the following requirement is met. The reflux of the mercuryand electrolyte must successively pass the electrode sumps and theelectrolytes, respectively, of the individual cells in that order ofsuccession which is opposite to the direction in which the indium isconveyed to the final stage through the succession of stages by means ofthe electrolytic action or specifically the electric field adjoining theelectrolysis stages.

In the case of a large number of stages, connected in series, laws ofdistribution prevail which are similar to those in the case offractional distillation in a distilling column or in the case ofcounter-current extraction in an exchange column. Hence, the lessprecious metals are selectively or preferably dissolved out of the anodemercury and deposited into the next cathode mercury to a lesser extentthan the indium, in consequence of which these less precious metalsbecome concentrated in the reflux flowing electrolyte. It is just thereverse in the with reflux mercury.

case of the more precious impurities which selectively or preferablyremain deposited in the anode mercury in the case of anodic dissolutionof indium out of such metal, while in the case of cathodic deposition ofindium, they too selectively or preferably enter or dissolve into thecathode mercury. Consequently, the mercury becomes enriched with themore precious impurities in the same way as the electrolyte becomesenriched with the less precious impurities.

For the purpose of continuously separating both the more precious andless precious impurities for an unlimited length of time, in accordancewith the process of the invention, parts of both the electrolyte and theelectrode metal are continuously recirculated from the final stage ofthe succession of electrolyses to the initial stage, and removed fromthe process at that point. A continuous reflux of the electrode metalcan be effected with particular ease using liquid metal. It is onaccount of the ready solubility of indium in mercury, that mercury ischosen as the metallic solvent electrode for indium.

The magnitude of the counter-currents of electrolyte and liquidelectrode materials which are necessary in order to remove the lessprecious and the more precious impurities, depends upon the number ofstages of electrolysis and upon the desired degree of purity of thefinally recovered indium metal. By increasing the number of stages, orby enlarging the counter-current, it will be appreciated that any suchimprovement in the purifying effect may be achieved. In so doing, it isnot necessary for the purpose of feeding the counter-current to feedinto the final stage pure indium for the electrode metal or pure indiumsalts for the electrolytes. Instead, it will suffice to add to a priorstage mercury as solvent for the indium metal and a pure acid, such as,for example hydrochloric acid, as electrolyte. Both of these substancescan be readily and economically produced by conventional methods withsuflicient purity for utilization in accordance with the invention, i.e.mercury with impurities less than 0.01 ppm. and hydrochloric aciddistilled in a quartz apparatus with a still higher degree of purity.Due to the side reactions which normally occur at the electrodes,especially in the intermediate stages, for example by cathodic evolutionof hydrogen, the electrolyte advantageously forms in the process itself.

It is convenient for the purpose of obtaining a good yield of metallicindium with an optimum degree of purity to adjust with respect to oneanother the power-loads in the individual stages of electrolysis withdue regard to the side reactions at the electrode, the feeding of thefirst stage with primary electrolyte containing indium in impure form,the feeding of the last stage but one with reflux electrolyte, and thatof the last amalgam circulation In this way the concentrations in theelectrolytes of the individual stages and in the amalgam electrode aremaintained at a convenient and suitable level.

It has been found to be convenient for the purposes of obtaining a highelectrolytic efficiency in the cathodic deposition of metals to use highindium concentrations in the electrolyte, i.e. concentrations not belowabout 50 grams of indium per litre of solution. However, as high aproportion as possible of indium primarily introduced into theelectrolysis is still capable of being cathodically deposited where theinitial electrolyte fed to the electrolysis possesses a very high indiumconcentration, i.e. from 400500 grams per litre, and where the primarystage is divided into several individual cells. In this manner, theindium content of the electrolyte may be reduced in stages to the finalconcentration of about 50 grams per litre, so that, even allowing forthe losses due to the countercurrent reflux and considering the decreasein volume which is caused by the removal of the metal and of the halogenfrom the electrolyte, about 80-90% of the indium, primarily fed to theelectrolysis with the initial elec- 6 trolyte, can be obtained in thefinal stage as purest metal. Of course, the electrolyte and the cathodeamalgam will flow through the series of cells in this primary stage inopposite directions in order to achieve a more efiicient exchange in themanner of a conventional extraction technique.

In the final deposition of the pure indium in the last stage ofelectrolysis, the difficulties which were encountered by the continuouscarrying out of the process due to the formation of dendrites at thecathode as well as through the necessity of frequently replacing thecathode are overcome in consequence of the present invention byoperating this stage at a temperature above the melting point of indium.Such operation offers as well all the advantages of a molten cathodewhereby the metal obtained electrolytically can be continuouslyseparated from the electrolysis, as for example via a Syphon. At such anelectrolytic temperature above the melting point of indium, of course,an aqueous electrolyte can no longer be used.

Surprisingly, however, it was discovered that in the case of indium, theanhydrous halides can readily be used in the molten stage as aneffective electrolyte in the last stage, this feature being contrary toexpected performance based upon the analogy of the behavior of aluminum,2. metal also recovered by electrolytic purification. In the presence ofmolten metallic indium, the metal present in the molten anhydrous indiumhalide has an average valence of 1.0 to 1.5. Therefore, a smaller amountof current is required for the deposition of a certain amount of indiumthan would be required were the electrolysis to be carried out inaqueous solution.

Tne mercury used for the production of the amalgam reflux, in accordancewith the over-all process of the invention, is conveyed to the mercurycathode of the penultimate stage of electrolysis or to the amalgam anodeof the last stage of electrolysis, or to the circulation between thesetwo stages whereas the electrolyte used for the production of theelectrolytic reflux is conveyed to the electrolyte of the penultimatestage only.

The melting point of low indium halide amounts to about 250 degrees C.and is sufficiently low in itself for use in the final stage inaccordance with the present invention. The melting point can be furtherlowered, nevertheless, by admixing several indium halides as well as byadmixing with the particular indium halide or mixture of halides one ormore alkali halides and/or alkali earth halides such as sodium,potassium, and lithium halides and/or calcium and barium halides.Consequently, the electrolysis in the final stage can be carried out ata temperature approximating 200 degrees C. The iowerin g of thetemperature of the final stage electrolysi might seem negligible exceptthat its advantage lies in the fact that in the final stage less mercuryis conveyed with the indium from the anode through the electrolyte tothe cathode, whereby less mercury will be found present as an impurityin the pure indium recovered from the last stage. The mercury alloyingconstituent in the deposited indium in the last stage can be reduced byemploying in the last or final stage a large anode surface and a smallcathode surface.

Any small amount of residual mercury, !which.rernains in the finallydeposited indium, can be readily removed by heating the tapped metal toabout 800 degrees C. in a vacuum, in a current of hydrogen, or in acurrent of another inert gas. In this manner, the indium metal obtainedwill be of highest purity and will be subject to no restrictions on itsuse for semi-conductor purposes.

'Indium halides are hygroscopic and at increased temperatures aresensitive to air. Thus, it is necessary for carrying out theelectrolysis in the last or final electrolytic cell to protect such cellfrom any access to air. This may be achieved by filling the cell with aninert gas. The simplest way of producing the halide which serves asaoaaavs electrolyte in the final stage, in this connection, is toprepare it in the cell itself by means of the action of halogen orhydrogen halide, which can be rarefied, if desired, with hydrogen orwith an inert gas, on the amalgam obtained in the course ofelectrolysis.

Where, in the final stage, smelting electrolysis is carried out using amolten indium salt as electrolyte and molten indium metal as cathode,the electrolytic countercurrent or reflux is carried out by introducingan electrolyte into the penultimate stage of electrolysis. It will beappreciated, in this connection, that for the purpose of preventingaqueous electrolytic traces of materials from penetrating into the finalstage from the aqueous electrolysis of the penultimate stage, separatordevices are employed, such as those of non-wettable synthetic material,particularly those of Teflon, a chemically inert synthetic material.Separators of the foregoing type may be conveniently interposed in theamalgam circulation stream in order to prevent short circuiting betweenmercury material used as anode and mercury material used as cathode.

The primary object of the final or last stage operated with a smeltingelectrolyte is to eflect the substantially complete separation of indiumfrom mercury added as an auxiliary material for conveying the indiumthrough the succession of cells. The separation of indium from theimpurities present in the starting material is already sufticientlycomplete at this point and is of no great concern. Nevertheless, thefinal stage may also be used for this purpose if fresh electrolyte iscontinuously produced from the anode amalgam in the final stage by theaction of halogen or halogen compounds, such as, for example, hydrogenhalides free from detrimental impurities, and if a corresponding portionof the electrolyte is conveyed into the penultimate stage, where it isdissolved partially in the electrolyte, partially in the amalgam, withdisproportionation occurring, i.e. In(l)halide is decomposed intoIn(III)halide solving in the electrolyte and metallic indium, solving inthe mercury.

The phase passage processes at the electrodes, in accordance with thepresent invention, take place sufliciently selectively up to very highcurrent densities. In order to avoid harmful concentrationpolarizations, detrimental to the purifying effect of the electrolysis,it is convenient to repump the electrolyte between the anode space andthe cathode space of each stage in a turbulent manner. The same holdstrue for passing the amalgam between the cathode space of one cell andthe anode space of the cell of the next adjacent stage. In this manner,current densities of up to 1,000 a./m. can be employed without beingnoticeably detrimental to the selectivity of the deposition ofimpurities.

In the primary stage, the anodic current density must be less if thecell is fed with an electrolyte of indium chloride which has beenproduced outside of the electrolysis. In this anodic evolution ofchlorine in the first electrolysis stage, it is convenient to useelectrodes of graph ite. Current density and anode wear amount to anorder of magnitude similar to that of the aqueous chlorinealkalihydrolysis of the conventional type. In using insoluble anodes withanodic evolution of chlorine in the first stage, it is convenient torenew continuously only the surface of the cathode amalgam by means ofrepumping while only little motion should be effected in the electrolytein order not to decrease the electrolytic efficiency at this point byreducing the chlorine at the cathode which ha been anodically evolvedand dissolved in the electrolyte. Impure indium metal itself can also beused as an anode in the first stage if desired. In such case, thefeeding of a foreign electrolyte may be partially or even completelyomitted since the indium metal will produce the required electrolyteconcentrations necessary for carrying out the over-all process of theinvention.

Where an electrolyte is employed in the intermediate stages having acomparatively high indium concentration, i.e. of more than 100 grams perlitre of indium, ad-

vantageously part of the indium is present in such aqueous solution witha valence of less than 3 due to the reaction with the cathode metal.Accordingly, with regard to trivalent indium, electrolytic etficiencieshigher than are reached. The acid added in the penultimate stage, inorder to produce the reflux is only partially decomposed by cathodicseparation of hydrogen such that the electrolyte remains suificientlyacid that hydrolysis will not occur even if there are high indiumconcentrations. Thus, for the purpose of obtaining high indiumconcentration in the electrolyte of the intermediate stages, it isconvenient to add the acid which is introduced for the purpose ofeffecting the electrolytic reflux with a sulficiently high concentrationpreferably from 3 to 8 normal. If hydrochloric acid is used, thesimplest way of attaining the required purity is by distillation or byintroducing gaseou HCl and adding water to the electrolyte of thepenultimate stage.

It will be appreciated that Where amalgam is used, it is normallyexposed to a certain danger of slagging in any process. In accordancewith the present invention, however, there is little such danger in thecounter-current electrolysis since the electrolyte is sufficiently acid.Nevertheless, if need be, such danger may be avoided by shutting off theelectrolytic cell from the atmosphere and filling such cell with aninert gas. Conveniently, the inert gas to be used may be the hydrogenevolved by means of the decomposition of the acid added for the purposeof producing the counter-current, if such hydrogen successively passesthrough the gas volumes or spaces of the individual cells. Forconvenience and effectiveness, such inert gas should be conveyed incounter-current flow to the direction of indium being conveyed throughthe electric field along the series of successive electrolysis stages.

The invention will be further illustrated in conjunction with theaccompanying example taken in connection with the accompanying drawing,but it will be appreciated that the invention is not to be limitedthereby.

Example The electrolytic deposition and redissolution of indium iseffected in four stages: 1, 2, 3, and 4. The first stage is divided intothree individual cells: 1a, 1b, and 1c. The impure initial electrolyte,an indium chloride solution having an indium content of 400 grams perlitre and a content of free hydrochloric acid of 10 grams per litre, isfed into the cell 1a at 5 and successively flows through the cells 1a,1b, and 1c such that the indium content is reduced to 50 grams perlitre, due to the deposition of indium from such solution at the mercurycathodes 6, 7 and 8 of these cells and the evolution of chlorine at thegraphite anodes 9, 10, and 11 disposed in opposing relation to thecathodes in the respective cells. The indium chloride electrolyte havinga concentration of 50 grams per litre leaves cell 10 at 12.

The cathode amalgam is successively drawn through cells 1c, 1b, and 1ain countercurrent direction to the flow of electrolyte through thecells. The cathode amalgam is conveyed by means of a pump 13 through anoverflow device 14, preventing a short circuit with the anode 15 ofstage 2, to which the cathode amalgam is circulated for again dissolvinganodically the indium from such cathode amalgam now maintained as ananode in stage 2. By means of a further overflow device 35, used forinsulation of the anode amalgam of stage 2, from the cathode amalgam ofcell 1c, the mercury is returned to the cathode of cell 10. A levelcontroller device 34 is interposed in the circulation of mercury betweenthe anode of stage 2 and cell 10 for purpose of maintaining constant theamount of amalgam in circulation.

The indium, dissolved in stage 2 from the anode 15 is conveyed via theelectrolyte 16 into the cathode amalgam 17. The stirrer device 18repumps in a turbulent manner the electrolyte of stage 2 between thecathode space and the anode space thereof, the partition Wall 19situated in stage 2 preventing the mixing of cathode mercury 17 withanode mercury 15.

However, there is an exchange between cathode mercury or amalgam 17 onthe one hand and anode amalgam or mercury 21 of stage 3 on the otherhand by means of the pump 20. The cathode amalgam 22 of stage 3 isexchanged in turn for the anode amalgam 24 of final stage 4 by means ofa circulating pump 23.

The electrolyte 25 of stage 4 consists of molten waterjfree indiumhalide and is kept at a temperature of 200-300 degrees C. The overflow36 of the amalgam circulation between stages 3 and 4 consists of asynthetic material,.non-wettable with water, and prevents thepenetraction of leaking traces of the electrolyte of stage 3 frompassing into stage 4. The cathode metal 26 in stage 4 consists of purestmolten indium, the amount of which is maintained constant by means ofthe outlet discharge at 27. The pure metal is discharged at 27 inproportion to the amount of increase of the cathode metal 26 caused ,bythe electrolysis. The pure metal discharged at 27 may be cast into barsat this point. The electrolytic countercurrent or reflux is produced bymeans of continuously adding hydrochloric acid, forexample, of aboutnormal to the electrolyte of stage 3 at 28. The electrolytes of stages3, 2, and 1b are in communication with one another via the pipes 29 and30. As a result of the arrangement .shown, no redifiusion of electrolyteis possible from cell 1b to stage 2 or from stage 2 to stage 3.Consequently, there flows through pipes 29 and 30 an electrolytic refluxwhich corresponds to the amount of hydrochloric acid .added at 28.

In stage 3, and to a lesser extent, in stage 2, an evolution of hydrogenoccurs at the amalgam electrode in addition to the cathodic depositionof indium. Therefore, the hydrochloric acid added at 28 for producingthe electrolytic reflux or counter-current, is converted on its waythrough stages 3 and 2 to an acidic indium chloride solution having anindium content of about 100 grams per litre. With regard to itsconcentration, the same roughly corresponds to that obtained by the feedelectrolyte, added at inlet 5, on its Way through tne first stage incell 1]). The reflux electrolyte leaves the apparatus at 12 along withthe feed electrolyte which has at this point been completelyelectrolyzed. The mercury serving to produce the amalgam counter-currentor reflux is added at 31 to the circulation between the anode amalgam 24of stage .4 and the cathode amalgam 22 of stage 3. The amount of suchamalgam is thereby increased, so that via the level controller and pumpdevice 32 a portion of amalgam is introduced into the circulationbetween stages 2 and 3. In the same way, a portion of amalgamcorresponding theretois introduced via the level controller device andpump .33-into the circulation between stages 1 and 2. Such amalgamcorresponding to the portion introduced at 31is eventually completelyremoved from the process during the circulation of amalgam betweenstages 2 and .1 by means of the level controller device 34. It will beappreciated, in this connection that a similar level controller deviceeffecting electric insulation as that of device 35 between anode andcathode 8 is provided as well between cathode 17 of stage 2 and anode 21of stage 3 and between cathode 22 of stage 3 and anode 24 of stage 4.

For the same purpose, device 36 is interposed in the circulation of pump23 between anode 24 of stage 4 and cathode 22 of stage 3, device 14 isinterposed in the circulation of pump 13 between anode 15 of stage 2 andcathode 6 of cell In, and device is interposed between anode 21 of stage3 and cathode 17 of stage 2. Moreover, in the conveying of mercury incounter-current reflux, the similar devices 32and 33 are provided, sothat in the entire arrangement no short-circuiting is possible duringthe passage of mercury from one cell to the next or from one electrodeto the next.

.Inthe amalgam which is discharged at 34, the proportion of silver,copper, and lead impurities to indium has aoaatws 10 changed to about 20times the value of that which is present in the initial electrolyte feedadded at 5. On the other hand, Zinc and iron are concentrated in thefinal electrolyte discharged at 12, such metals being originally presentin the impure indium feed added at 5 to cell 1 1. The only impuritiesdetectable in the purest indium discharged at 27 are small amounts ofmercury. The indium is freed from such mercury prior to casting, bymeans of heating the same to 800 degrees C. in a current of hydrogenwhereby the mercury is separated.

Accordingly, the present invention relates to an improvement in theprocess for the continuous electrolytic production of metallic indium ofhighest purity from indium salt solutions'by means of a succession ofstages of individual electrolyses in which the indium cathodicallydeposited from the electrolyte salt solution into a mercury electrode ina previous stage is anodically dissolved from the mercury into a furtherelectrolyte salt solution and in turn cathodically deposited from saidsolution into a further mercury electrode inthe next stage. Suchimprovement essentially contemplates continuously introducing mercuryand electrolyte into the penultimate stage of a succession of stages ofindividual electrolyses, passing said electrolyte successively forwardtherefrom to the beginning stage of said succession against theelectrolytic flow of indium through said stages and continuouslywithdrawing electrolyte from the beginning stage corresponding to thatintroduced into the penultimate stage, with respect to the penultimatestage and previous stages excluding the beginning stage circulatingmercury between the anode of one stage and the cathode of the nextprevious stage and passing from said penultimate stage successive- 1yforward against the electrolytic flow of indium through said stages, atleast a portion of said mercury corresponding to that introduced intothe penultimate stagefrom the circulating mercury between the anode ofone stage and the cathode of the next previous stage to the circulatingmercury between the anode of said next previous stage and the cathode ofthe stage before that, with respect to said beginning stage circulatingthe mercury between the cathode of said beginning stage and the anode ofthe next successive stage after the beginning stage and passing .aportion of mercury, corresponding to that portion introduced into saidpenultimate stage, from the mercury circulating between the cathode ofthe next successivestage after the beginning stage and the anode of thenext stage after that to the mercury circulating between the anode ofsaid next successive stage after the beginning stage and the cathode ofthe beginning stage, continuously withdrawing mercury from the beginningstage corresponding to that introduced into the penultimate'stage,maintaining a molten indium salt as electrolyte and molten pure indiummetal as cathode in the end stage of said succession, circulatingmercury between the cathode of said penultimate stage and the anode ofsaid end stage, said mercury introduced into said penultimate stage,being introduced into said last-mentioned circulating mercury, addingimpure indium salt solution to the said beginning stage and recoveringfrom said molten cathode in pure molten form indium electrolyticallydeposited from said molten electrolyte into said molten indium purecathode.

What is claimed is:

1. Process for the continuous electrolytic production of metallic indiumof the highest purity from indium salt solution which compriseselectrolyzing an impure indium salt solution in a first stageelectrolysis using mercury as cathode whereby to form a first amalgam ofmercury and electrolytically separated metallic indium, passingsaidamalgam to an intermediate stage electrolysis for electrolysis thereinusing an indium salt solution as electrolyte, said amalgam as anode andadditional mercury as cathode whereby to dissolve electrolytically saidmetallic indium from said amalgam into said electrolyteand in turnelectrolytically deposit said indium in said additional mercury ascathode to form further amalgam, passing electrolyte from saidintermediate stage electrolysis to said first stage electrolysis forcombining with said impure salt solution for further electrolysistherein, passing mercury remaining upon electrolytic removal of metallicin dium from said first amalgam as anode from said intermediate stage tosaid first stage for use therein as cathode, passing said furtheramalgam to a last stage electrolysis using said further amalgam asanode, molten indium salt as electrolyte and molten pure indium metal ascathode for dissolving electrolytically said metallic indium from saidfurther amalgam into said molten indium salt and in turnelectrolytically depositing said indium in said molten indium metal ascathode, and recovering pure indium metal from said last stage.

2. Process according to claim 1 wherein the electrolyte in the first andintermediate stages is indium halide solution and that in the last stageis molten water-free indium halide.

3. Process according to claim 2 wherein the last stage electrolyte ismaintained at a temperature above its own fusion point and above thefusion point of the indium.

4. Process according to claim 3 wherein the last stage electrolyte is alow-melting mixture of indium halides.

5. Process according to claim 4 wherein said low-melting mixtureadditionally includes a member selected from the group consisting ofalkali halide, alkaline earth halide, and mixtures thereof.

6. Process according to claim 1 wherein the electrolyte is agitated insaid intermediate stage.

7. Process according to claim 1 wherein the passing of amalgam from onestage to the other is carried out by pumping in a lively manner.

8. Process according to claim 1 wherein the electrolyte of the firststage is produced by anodic dissolution of impure indium metal.

9. Process according to claim 1 wherein the electrolyte is added to theintermediate stage in the form of pure aqueous hydrohalic acid having aconcentration of 3 to 8 normal and mercury substantially free fromimpurities is also added to said intermediate stage.

10. Process according to claim 1 wherein the electrolyte in the laststage is produced by the action of a member selected from the groupconsisting of halogen and hydrogen halide gas on said further amalgampassed to said last stage.

11. Process according to claim 10 wherein said gas is rarefied by aninert gas.

12. Process according to claim 10 wherein said gas is rarefied byhydrogen.

13. Process according to claim 1 wherein said first stage includes atleast two individual electrolysis cells in series, the electrolyte andamalgam being passed through the series of cells in this stage inopposite directions.

14. Process for the continuous electrolytic production of metallicindium of the highest purity from indium salt solutions which compriseselectrolyzing an impure indium salt solution being passed to a firststage electrolysis using mercury as cathode whereby to form a firstamalgam of mercury and electrolytically separated metallic indium,passing said amalgam to an intermediate stage electrolysis forelectrolysis therein using an indium salt solution as electrolyte, saidfirst amalgam as anode and additional mercury as cathode whereby todissolve electrolytically said metallic indium from said first amalgaminto said electrolyte and in turn electrolytically deposit at least aportion of said indium in said additional mercury as cathode to formfurther amalgam, passing a portion of said indium salt solution enrichedby a portion of said metallic indium dissolved electrolytically thereinfrom said intermediate stage electrolysis back to said first stageelectrolysis for combining with said impure indium salt solution forfurther electrolysis therein, passing mercury remaining uponelectrolytic removal in said intermediate stage electrolysis of metallicindium from said first amalgam as anode from said intermediate stageback to said first stage for use therein as cathode, passing saidfurther amalgam to a last stage electrolysis using said further amalgamas anode, molten indium salt as electrolyte and molten pure indium metalas cathode for dissolving electrolytically said metallic indium fromsaid further amalgam into said molten indium salt and in turnelectrolytically depositing said indium in said molten indium metal ascathode, recovering a portion of pure indium metal corresponding to thatproduced in said electrolysis from said last stage, recovering a portionof electrolyte from said first stage, adding fresh electrolyte to saidintermediate stage to replace that passed to said first stage andcorresponding to that recovered from said first stage, recovering aportion of mercury being passed from said intermediate stage to saidfirst stage, and adding fresh mercury to said intermediate stagecorresponding to that recovered from said first stage.

15. Process according to to claim 14 wherein the fresh electrolyte addedto said intermediate stage is in the form of a pure aqueous hydrohalicacid having a concentration of 3 to 8 normal.

16. Process according to claim 14 wherein the fresh mercury free fromimpurities is added to the additional mercury used as cathode in saidintermediate stage.

17. Process according to claim 16 wherein a portion of the mercury beingpassed from the cathode of said intermediate stage to the anode of saidlast stage is withdrawn and passed to the anode of said intermediatestage.

18. Process according to claim 14 wherein a series of individual cellsis used in said first stage and in said intermediate stage, said mercurypassed to said first stage from said intermediate stage passinginversely successively through the cells of said first stage in oppositedirection to the flow of electrolyte from said intermediate stage andthe flow of impure indium salt solution through the cells of said firststage, said fresh electrolyte passing into and through the cells of saidintermediate stage in the same direction as the flow through the cellsof said first stage, the electrolytic passage of indium from theelectrolyte in the cells of said first stage by said first amalgam tothe electrolyte in the first cell of said intermediate stage and fromthe electrolyte of said first cell successively through the remainingcells of said intermediate stage by means of said additional mercury andfurther amalgam being in opposite direction to the flow of said freshelectrolyte, whereby a portion of pure indium will be continuouslydeposited at the cathode of the last stage, and a portion of indiumpresent in the oppositely flowing electrolyte will be continuouslypassed back therewith through the successive cells in said intermediatestage and through at least one cell of said first stage and will bewithdrawn with the electrolyte recovered from said first stage.

19. In the process for the continuous electrolytic production ofmetallic indium of highest purity form indium salt solutions by means ofa succession of stages of individual electrolyses, in which the indiumcathodically deposited from the electrolyte salt solution into a mercuryelectrode in a previous stage is anodically dissolved from the mercuryinto a further electrolyte salt solution and in turn cathodicallydeposited from said solution into a further mercury electrode in thenext stage, the improvement which comprises continuously introducingmercury and electrolyte into the end stage of a succession of stages ofindividual electrolyses, passing said electrolyte successively forwardto the beginning stage of said succession and continuously withdrawingelectrolyte from one of the earlier previous stages corresponding tothat added to the end stage, in the case of a portion of the latersuccessive stages including the end stage circulating mercury betweenthe anode of one stage and the cathode of the next previous stage, inthe case of said portion of later successive stages passing at least aportion of said mercury, corresponding to that introduced into the endstage, from the circulating mercury between the anode of one stage andthe cathode of the next previous stage to the circulating mercurybetween the anode of said next previous stage and the cathode of thestage before that, in the case of the remaining portion of earliersuccessive stages including the beginning stage circulating mercurysuccessively through the cathodes of said stages in opposite directionto the flow of electrolyte therethrough and in turn through the anode ofthe next successive stage corresponding to the first of said portion oflater successive stages including the end stage, passing a portion ofmercury, corresponding to that portion introduced into the end stage,from the mercury circulating between the cathode of the first and theanode of the next successive stage of said portion of later successivestages including the end stage to the last of said remaining portion ofearlier successive stages including the beginning stage, andcontinuously withdrawing mercury from one of the remaining portion ofearlier successive stages corresponding to that added to the end stage.

20. Improvement according to claim 19 wherein a further recovery stageis provided having molten indium salt as electrolyte and molten pureindium metal as cathode, and amalgam as anode, the anode amalgam beingcirculated between the anode of said further recovery stage and thecathode of the end stage of said succession of stages, whereby indiumelectrolytically deposited from said molten electrolyte into said moltenpure indium cathode is recovered from said molten cathode in pure moltenform.

211. In the process for the continuous electrolytic production ofmetallic indium of highest purity from indium salt solutions by means ofa succession of stages of individual electrolyses, in which the indiumcathodically deposited from the electrolyte salt solution into a mercuryelectrode in a previous stage is anodically dissolved from the mercuryinto a further electrolyte salt solution and in turn cathodicallydeposited from said solution into a further mercury electrode in thenext stage, the improvement which comprises continuously introducingmercury and electrolyte both free from detrimental impurities into thepenultimate stage of a succession of stages of individual electrolyses,passing said electrolyte successively forward therefrom to the beginningstage of said succession against the electrolytic flow of indium throughsaid stages and continuously Withdrawing electrolyte from the beginningstage corresponding to that introduced into the penultimate stage, withrespect to the penultimate stage and previous stages excluding thebeginning stage circulating mercury between the anode of one stage andthe cathode of the next previous stage and passing from said penultimatestage successively forward against the electrolytic flow of indiumthrough said stages at least a portion of said mercury corresponding tothat introduced into the penultimate stage from the circulating mercurybetween the anode of one stage and the cathode of the next previousstage to the circulating mercury between the anode of said next previousstage and the cathode of the stage before that, with respect to saidbeginning stage circulating mercury between the cathode of saidbeginning stage and the anode of the next successive stage after thebeginning stage and passing a portion of mercury, corresponding to thatportion introduced into said penultimate stage, from the mercurycirculating between the cathode of the next successive stage after thebeginning stage and the anode of the next stage after that to themercury circulating between the anode of said next successive stageafter the beginning stage and the cathode of the beginning stage,continuously withdrawing mercury from the beginning stage correspondingto that introduced into the penultimate stage, maintaining a moltenindium salt as electrolyte and molten pure indium metal as cathode inthe end stage of said succession, circulating mercury between thecathode of said penultimate stage and the anode of said end stage, saidmercury introduced into said penultimate stage being introduced intosaid lastmentioned circulating mercury, adding impure indium saltsolution to said beginning stage, and recovering from said moltencathode in pure molten form indium electrolytically deposited from saidmolten electrolyte into said molten pure indium cathode.

22. Improvement according to claim 21 where said impure indium salt andsaid molten indium salt is indium halide and said electrolytecontinuously introduced into said penultimate stage is pure aqueoushydrohalic acid having a concentration of 3 to 8 normal.

References Cited in the file of this patent UNITED STATES PATENTS2,382,434 McNitt Aug. 14, 1945 FOREIGN PATENTS 800,153 Great BritainAug. 20, 1958 581,668 Canada Aug. 18, 1959 OTHER REFERENCESIahresberichte uber die Fortschritte der Chemie, Theil 1 (1888), pages388-389.

1. PROCESS FOR THE CONTINUOUS ELECTROLYTIC PRODUCTION OF METALLIC INDIUMOF THE HIGHEST PURITY FROM INDIUM SALT SOLUTION WHICH COMPRISESELECTROLYZING AN IMPURE INDIUM SALT SOLUTION IN A FIRST STAGEELECTROLYSIS USING MERCURY AS CATHODE WHEREBY TO FORM A FIRST AMALGAM OFMERCURY AND ELECTROLYTICALLY SEPARATED METALLIC INDIUM, PASSING SAIDAMALGEN TO AN INTERMEDIATE STAGE ELECTROLYSIS FOR ELECTROLYSIS THEREINUSING AN INDIUM SALT SOLUTION AS ELECTROLYTE, SAID AMALGAM AS ANODE ANDADDITIONAL MERCURY AS CATHODE WHEREBY TO DISSOLVE ELECTROLYTICALLY SAIDMETALLIC INDIUM FROM SAID AMALGAM INTO SAID ELECTROLYTE AND IN TURNELECTROLYTICALLY DEPOSITED SAID INDIUM IN SAID ADDITIONAL MERCURY ASCATHODE TO FORM FURTHER AMALGAM, PASSING ELECTROLYTE FROM SAIDINTERMEDIATE STAGE ELECTROLYSIS TO SAID FIRST STAGE ELECTROLYSIS FORCOMIBINING WITH SAID IMPURE SALT SOLUTION FOR FURTHER ELECTROLYSISTHEREIN, PASSING MERCURY REMAINING UPON ELECTROLYTIC REMOVAL OF METALLICIN-